Vaisesika Sūtra 3.2.01

आत्मेन्द्रियार्थसंन्निकर्षे ज्ञानस्य भावोऽभावश्च मनसो लिङ्गम् ॥१॥

ātma-indriya-artha-saṁnikarṣe jñānasya bhāvaḥ-abhāvaś-ca manasaḥ liṅgam ॥1॥

[आत्मेन्द्रियार्थसंन्निकर्षे (ātma-indriya-artha-saṁnikarṣe) - coming together of ātma, senses, and objects; ज्ञानस्य (jñānasya) - of knowledge; भावः (bhāvaḥ) - presence; अभावः (abhāvaḥ) - absence; च (ca) - and; मनसः (manasaḥ) - of the mind; लिङ्गम् (liṅgam) - indication;]

The presence or lack of knowledge arising from the coming together of ātma, senses, and objects is the indication of mind (manas).